1. Field of the Invention
The invention refers to the field of electronic reproduction technology and is directed to a method and to an apparatus for white balancing of the scanning elements of reproduction apparatus, particularly of black/white scanners and color scanners for the production of printing forms.
2. Description of the Prior Art
In a black/white scanner, a black-white original arranged on a rotating scanner drum is scanned point-by-point and line-by-line by a scanner light source and the scan light modulated by the tonal values of the scanned picture elements of the original in a scanner element is converted into an image signal by an opto-electrical transducer, for example by a photomultiplier, this image signal representing the tonal values between "black" and "white". The image signal is then modified according to a prescribed gradation curve whose corner points mark the tonal values "black" and "white" and whose course between the corner points is dependent on the printing parameters for the later printing and on the desired changes in tonal value of the reproduction in comparison to the original. The modified image signal controls the brightness of a recording element that exposes a film point-by-point and line-by-line. The exposed and developed film serves for the production of the printing form for the later printing of the original.
In a color scanner, the scan light acquired by the point-by-point and line-by-line scanning of a color original is split into three color components with dichroitic filters, these color components being converted into color signals by three-opto-electrical transducers. The color signals are likewise modified according to gradation curves and are then converted into the color separation signals in a color correction computer for recording the color separations, the printing forms for the later multi-color printing of the original being prepared from these color separations.
For standardizing the reproduction process, fixed reference signal values referred to as white level and black level are allocated to the corner points of the gradation curve. With reference to the white level, however, what is referred to as a white balancing must see to it that the maximum scan light proceeding from a white normal is always converted into an image signal value in the photomultiplier that corresponds to the normed white level, this being achieved by an appropriate adjustment of the sensitivity or, respectively, of the gain of the opto-electrical transducer.
The brightest image locations (white points) of the originals to be scanned are usually used as white normals in practice, whereby the scan light proceeding from the white point corresponds to the light of the scanner light source attenuated by the respective density value of the white point.
Since the scan light proceeding from the respective white point varies from original to original and dependent on the type of original carrier (film material given transparency originals and photographic paper given opaque originals) and since the sensitivity of the opto-electrical transducers that are employed cannot be kept constant over a longer time span, a white balancing is fundamentally carried out in practice before every original scanning. This white balancing is involved particularly in color scanners since at least three opto-electrical transducers must be balanced therein. In order to keep the time required for the white balancing short in comparison to the actual scan time for the originals, attempts have been made to optimally automate the white balancing.
For example, DE-A-No. 25 45 961 which corresponds to U.S. Pat. No. 4,136,360 discloses such an automatic white balancing. Therein, the scanner element is positioned to the respective white point of the original to be scanned and the scan light proceeding from the white point is converted into an image signal value in the photomultiplier as actual value for a control means. In the control means, the measured actual value is compared to a rated value corresponding to the white level and the sensitivity or, respectively, the gain of the photomultiplier is modified via the supply high-voltage until the actual value corresponds to the rated value.
The known method has the disadvantage that the scan light corresponding to the white normal must be generated by scanning corresponding white points of originals. In order to achieve a precise and reproducible white balancing, the operator must position the scanner element to the white point with optimum precision, even given repetitions of the balancing procedure, this being involved and time-consuming. Added thereto is that a white passage in the original suitable as white point is frequently not present in a colored original or that the brightest passage of the original has a color cast that is not to be compensated in the reproduction.
It is likewise already known to generate the white normal required for the white balancing not by scanning a white point of an original but by scanning a graduated optical wedge applied on the scanner drum, whereby the degree of attenuation of the light of the scanner light source is determined by the selected density value of the graduated optical wedge for simulating the scan light of a white point. The employment of a graduated optical wedge in white balancing is involved and also has the disadvantage that the white balancing can be faulty due to damage to or contamination of the graduated optical wedge.
It is also known to pivot neutral density filters into the beam path of the scanner element for light attenuation in the white balancing. Since even good neutral density filters exhibit differences in spectral transmission, disturbing color displacements can occur given white balancing in color scanners, whereby it is particularly the reproduction of neutral tones that is falsified. The differences in spectral transmission of the grey filters cannot be eliminated by calibration measurements.
It is thus the object of the invention to avoid the said disadvantages of a traditional white balancing and to recite a method and an apparatus with which a fast, more precise and chromatically neutral white balancing can be executed without scanning a white point in an original and without employing graduated optical wedges and neutral density filters.
With respect to the method, this object is achieved by the features of claims 1 and 22 and, with respect to the apparatus, is achieved by the features of claims 14 and 34.
DE-A-No. 34 32 176 which corresponds to U.S. Pat. No. 4,563,707 in fact also discloses a balancing method for the scanner element of a reproduction apparatus; this, however, is based on a different object. What is involved therein is balancing given different apertures of the scanner diaphragm that are selected in accord with the desired finenesses of scanning and measures that are intended to avoid the destruction of the photomultipliers given an excessively intense light incidence. See also U.S. Pat. Nos. 4,647,981 and 4,639,787.